Bluetooth is one wireless technology standard for exchanging data between fixed and mobile devices in a wireless communication network (called a Bluetooth network when Bluetooth is employed). A master or central Bluetooth device may communicate with multiple peripheral Bluetooth devices within the Bluetooth network.
In general, the operation of any radio communication network (e.g., in a Bluetooth Low Energy (BLE) network) is limited by at least the following: the signal-to-noise ratio (SNR) of the radio link(s); multiple access interference from other devices in the same network; and/or external interference. For example, BLE operates in the unlicensed 2.4 GHz band (ISM band from 2.4 to 2.485 GHz), in which it must co-exist with various other communication networks, such as WiFi. It may hence be exposed to, and must be able to operate in the presence of, interference from co-existing networks.
Further, advances in BLE technology may increase multiple access interference from other devices within a BLE network. In its current form, one single transmission rate, namely 1 Mbps physical layer bitrate, is supported but work is ongoing to add support for both higher and lower bitrates. The higher bitrate mode is referred to as the 2 Mbps mode, whereas the lower bitrates go under the common name BLE long range (LR), which supports bitrates down to 125 kbps on the physical layer. As the name implies the BLE LR mode extends the transmission range of the BLE communication networks. With support for an increased transmission range, the number of devices participating in a network may grow since, with an increased range, each device may reach out to a larger number of peer devices. In addition, using a lower transmission rate implies that it takes longer time to transmit data over the air.
The BLE link layer specifies that there are 40 channels available for communication. Three of those channels are advertisement channels used, e.g., for broadcasting of data and setting up data connections, and 37 are data channels. The BLE link layer channel map 100 is depicted in FIG. 1. The advertising channels are considered contention-based and the data channels are considered contention-free.
FIG. 2 depicts the current connection setup in BLE. Connection setup in BLE is initiated by a peripheral device connecting to a central device. When such a peripheral device has a need to communicate with the central device, it sends an advertisement message 200 destined to the central device (it may be directed to a single central device or open such that any central device may answer). This advertisement message 200 is transmitted using one of the link layer advertisement channels 270. If the central device receives the advertisement message it will answer with a connection request message 210, which includes connection setup parameters required to start the data exchange. The connection request message is transmitted over the same channel as the advertisement message 200 was received on and a given time after the reception of the advertisement message 200. The peripheral device hence knows at which channel and at what time the answer from the central device, i.e., the connection request, is expected. A typical BLE transaction, including the connection setup procedure, is depicted in FIG. 2. FIG. 2 shows that once the connect request message 210 is sent, the central device also transmits a connection establishment message 220 on a different channel; namely, a so called data channel 280. The peripheral device can then begin transmitting data to the central device on the data channel 280. The peripheral device also informs the central device when the peripheral device wishes to terminate the connection in a termination message 250. The central device acknowledges 250 the receipt of data and acknowledges 260 the termination message. In other situations it may also be the central device that terminates the connection.
A frequency hopping spread spectrum (FHSS) radio transmission process may be used for transmission over the data channels. Under FHSS, the transmission regularly hops between different data channels on different frequencies according to a predetermined code. The receiver of the transmission must also receive on the same frequencies using the same frequency hopping sequence. Thus, if there is for instance channel noise localized to specific frequencies as opposed to spread across the frequencies, the effect of this disturbance on communication is minimized.
The setup procedure is well designed to handle situations in which the network is SNR-limited, and to some extent also the handling of external interference. As the advertisement message had an SNR sufficient for successful reception, it is also reasonable to assume that the answer in the opposite direction has a good chance of being successfully received; the channel is reciprocal and typically similar transmit powers are used. Also, if the reception of the advertisement message was not limited by external interference, it is a decent assumption that the interference situation in the opposite direction is such that also the connection request can be received. Finally, the fact that the two messages are transferred on the same channel implies that the radio hardware does not need to switch frequency, which simplifies the implementation.